1. Field of the Invention
This invention is in the field of comminution of brittle products characterized by a relatively low specific energy consumption with a high degree of comminution and a low apparatus outlay. It involves the immediately sequential steps of unit grain comminution followed by product bed comminution all occurring in the same roller gap of a pressure roller machine.
2. Description of the Prior Art
In pressure comminution of brittle grinding stock in the roller gap of a roller machine, the grinding stock is crushed or comminuted by means of compressive stress. Crackers or roll-jaw crushers are usually operated on the principles of unit grain comminution. The feed is uniformly distributed over the width of the roller and is supplied to the rollers in such slight amounts that the particles of the feed do not crush each other in the roller gap. Cracking or crushing between the rollers is carried out, in effect, grain to grain. This operating mode is standard when as small as possible a proportion of superfine material is desired in the product, i.e., there must be a tight grain size distribution or a "steep" cumulative size distribution curve. With plain roller machines, the degree of comminution is limited to about 1:4 ("Ullmanns Encyklopadie der technischen Chemie", Fourth Edition 1972, Volume 2, page 12, left-hand column).
It is also known to operate roller mills according to the principle of product bed comminution. In this type of comminution, the feed is supplied to the roller gap in such a large quantity that the product to be comminuted is drawn in between the rollers and presses the rollers apart, so that the particles of the feed mutually crush one another in the roller gap to produce an agglomerated product bed. A gap width of the rollers resiliently pressed against each other greater than the particle size of the feed is maintained. This operating mode of product bed comminution in roller mills is often referred to in English as "choke feeding" (see, for example, "Handbook of Mineral Dressing" by Taggart, New York, January 1948, page 4-57, paragraph 4).
The distinction between unit or individual grain crushing and product bed crushing is also discussed in German OS No. 1 757 093 which serves to explain the state of the art.
When a high degree of comminution or high fineness of the grinding stock are to be achieved, multi-stage grinding methods such as pre-comminution, mean comminution and fine comminution have been employed. An attempt was made to match the stress conditions in each of the stages or in the comminution machines to the size of the grinding stock particles to be comminuted in order to reduce the energy consumption of the overall process. The multi-stage grinding methods with comminution machines connected in series, however, produce a high overall specific energy consumption as well as high overall capital costs. Ball mills are distinguished by a high degree of comminution and can grind chunks of cement clinker to cement fineness but the specific energy of the ball mills, however, is high.